A conventionally known method of improving the conductivity of polyimide is to mix a conductive filler in the polyimide (for convenience, the method will be referred to as “method I”). The “conductive filler,” or simply “filler,” is typically one of a variety of base materials coated with carbon, graphite, metal particles, indium tin oxide, or another conductive material to provide conductivity.
Improvement of conductivity by method I, however, raises following problems. First, most polyimide films obtained are of inferior mechanical properties. Next, conductive fillers (filler) obtained by method I show a very low resistivity (no more than about 103 Ω·cm in resistance) and present extreme difficulties in controlling the resistance value in the semiconducting region. Especially, the fillers are difficult to enhance reproducibility of surface resistivity and volume resistivity and reduce their in-plane irregularities. Also, polyimide films obtained by method I show a resistivity which has relatively large dependence on measurement voltage.
As a similar technique, Japanese Unexamined Patent Application 1-146957/1989 (Tokukaihei 1-146957; published on 8 Jun. 1989) discloses a resin composition filled with metal oxide fine powder (filler) and carbon black (for convenience, the technique will be referred to as “method II”).
Method II however is designed specifically to manufacture a semiconducting resin composition suitable for a sleeve of a developing roll for use in electronic photographic image forming apparatus of a single component developing system. In other words, the technique aims to obtain a semiconducting resin composition which exhibits desirable properties as a sleeve. Such properties are hard to achieve using either one of a highly conductive filler or carbon black; both of them are therefore used.
In fact, in method II, emphasis is only on adjustment of the volume resistance, which makes it difficult to properly adjust the surface resistance when a film is made of the resin. When method II is applied to a polyimide film, the resultant polyimide film has poor surface resistivity reproducibility as was the case with method I.
As a technique successfully addressing problems of these methods, for example, Japanese Unexamined Patent Applications 8-259810/1996 (Tokukaihei 8-259810; published on 8 Oct. 1996) and 8-259709/1996 (Tokukaihei 8-259709; published on 8 Oct. 1996) disclose a method whereby polyaniline is polymer-blended with polyimide to impart conductivity (for convenience, the method will be referred to as “method III”).
Method III have following problems. First, polyaniline's conductivity is ion conductivity, and its resistance value therefore is largely susceptible to environmental conditions. In addition, polyaniline has yet to establish industrially sufficient manufacturing productivity and is very costly to use as a polymer blend.
Apart from method III, for example, Japanese Unexamined Patent Applications 11-279437/1999 (Tokukaihei 11-279437; published on 12 Oct. 1999), 2000-207959 (Tokukai 2000-207959; published on 28 Jul. 2000), and 2001-2954 (Tokukai 2001-2954; published on 9 Jan. 2001) discloses preparation and use of a coating liquid in the formation of a conductive metal oxide film on a non-conductive base to provide conductivity (for convenience, will be referred to as “method IV”).
Method IV however only forms a conductive film on the base surface and is unable to reduce the base's volume resistance.
These conventional methods thus present difficulties in precisely controlling the surface and volume resistance of the polyimide film, making resistance less dependent on voltage, and enhancing its elongation rate and mechanical properties.